"gather[ed] particular recognition for the film's unique color processing, which rendered most of the film in black and white but retained or added coloring for select objects"
Further, they say,
Colorization is used on certain subjects in a scene, such as Devon Aoki's red-and-blue clothing; Alexis Bledel's blue eyes and red blood; Michael Clarke Duncan's golden eye; Rutger Hauer's green eyes; Jaime King's red dress and blonde hair; Clive Owen's red Converse shoes and Cadillac; Mickey Rourke's red blood and orange prescription pill container; Marley Shelton's green eyes, red dress, and red lips; Nick Stahl's yellow face and body; and Elijah Wood's white glasses. Much of the blood in the film also has a striking glow to it. The film was color-corrected digitally and, as in film noir tradition, treated for heightened contrast so as to more clearly separate blacks and whites. This was done not only to give a more film-noir look, but also to make it appear more like the original comic.
Many of its reviews talk about how "visually groundbreaking"[3] the movie was. To me, the use of black and white, touched with mostly red (and sometimes yellow and blue), was particularly visually appealing. Here are some shots from the movie:
Remember, this was in 2005. My vote for best color manipulation in a film.
[1] not to be confused with SimCity (video game)
[2][3] Page on rottentomatoes.com